Hitherto, there have been widely used magnetic recording media of the coated type prepared by dispersing a powdered magnetic material such as the magnetic powder or ferromagnetic alloy powder of gamma-Fe.sub.2 O.sub.3, gamma-Fe.sub.2 O.sub.3 doped with Co, Fe.sub.3 O.sub.4, Fe.sub.3 O.sub.4 doped with Co, a berthollide compound of gamma-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, CrO.sub.2, or the like in an organic binder such as a vinyl chloride-vinyl acetate copolymer, a styrene-butadiene copolymer, an epoxy resin, a polyurethane, or the like and then coating the resulting dispersion on a non-magnetic substrate or support and drying it. In recent years, with an increasing demand for high density recording, keen attention has been given to so-called non-binder type magnetic recording media having as a binderless magnetic recording layer, a ferromagnetic metal film formed by a vapor deposition method such as vacuum deposition, sputtering, or ion plating or by a plating method such as electroplating or electroless plating, and various efforts have been made to put them into practice.
Since the conventional coated-type magnetic recording media comprise a magnetic material, mainly composed of a metal oxide, which has a smaller saturated magnetization than that of a ferromagnetic metal, reduction in the thickness of the magnetic layer, which is necessary for high density recording, leads to a lowering in the signal output, and therefore the limit of thickness reduction has been reached already. In addition, the conventional coated-type magnetic recording media have the defects that the processes for manufacturing them are complicated and require large incidental facilities for solvent recovery or prevention of environmental pollution. In binderless type magnetic recording media, the ferromagnetic metal film having saturated magnetization larger than that of the above-mentioned metal oxide is Aformed without a non-magnetic material such as a binder, so that the magnetic recording media have advantage that the magnetic layer thereof can be made ultra-thin for high density recording and also the process for manufacturing them can be simplified.
High coercive force and reduction in thickness are proposed theoretically and experimentally in order to meet the requirements for magnetic recording media for high density recording, and much is hoped for binderless type magnetic recording media, wherein the thickness of the magnetic recording layer is easily reduced to a thickness one order as small as that of a coated-type media and wherein the saturated magnetic flux density is high.
In particular, a method for manufacturing magnetic recording media by vacuum deposition is very advantageous because not only does it not require a treatment for waste liquids which is needed in the case of plating, but also it is a simple process and can be operated at a high deposition velocity. Examples of the conventional method for manufacturing a magnetic film having a coercive force and squareness property suited for magnetic recording media by vapor deposition include an oblique vapor deposition method as described in, e.g., U.S. Pat. Nos. 3,342,632 and 3,342,633.
In actually manufacturing a magnetic recording medium by providing a ferromagnetic metal film on a tape-shaped substrate by oblique vapor deposition, a tape-shaped substrate is conveyed along a cooling can and a vapor stream of a ferromagnetic metal material evaporated from an evaporation source is allowed to collide with the moving tape-shaped substrate obliquely, i.e., at a predetermined angle of incidence, to vapor deposit the magnetic metal material thereon. In this process, the larger the incident angle of the vapor stream upon the substrate, the larger the coercive force of the resulting magnetic film but, at the same time, the lower the vapor deposition efficiency, which is a problem in the production thereof. Introduction of oxygen gas in oblique vapor deposition was proposed for forming the magnetic film having a high coercive force at a relatively small incident angle. For example, a method in which a tape-shaped substrate is conveyed so that the incident angle (.theta.) of the vapor stream upon the substrate may change continuously from a high incident angle (.theta.max) to a low incident angle (.theta.min) and, at the same time, an oxidative gas is introduced in the vicinity of the low incident angle (.theta.min) of the vapor stream, is disclosed in U.S. Pat. No. 4,477,489. According to this method, a magnetic film having a high coercive force can be obtained, but the magnetic film is defective in that it deteriorates the squareness property.
A method for obliquely vapor depositing a ferromagnetic metal material on a substrate with the introduction of carbon dioxide gas into a vacuum chamber for improving the deteriorated squareness property, is disclosed in Japanese Patent Application (OPI) No. 208937/1983 (the term "OPI" used herein means a "published unexamined Japanese Patent Application").
Further, magnetic recording media obtained by a oblique vapor deposition method while introducing an oxidative gas or carbon dioxide gas have insufficient film strength as a magnetic film, so that they have insufficient durability when the magnetic tape travels in a VTR or the like, and curling of the tape often occurs with the magnetic film inside. Elimination of these defects is desired.
In addition, a method in which cobalt is obliquely vapor-deposited on a polyester film substrate while introducing nitrogen, argon, or hellium gas into the vacuum vessel where the vapor deposition is conducted in order to improve magnetic characteristics of the magnetic recording media is disclosed in "Shinku" (Vaccum), Vol. 24, No. 4, p. 243-245. However, no improvement in the magnetic characteristics of the magnetic film is achieved in the case where a magnetic tape is prepared by an oblique vapor deposition method under the conditions that a continuous tape-shaped substrate is continuously carried and the incident angle of vapor stream upon the substrate is changed continuously, with introduction of argon (Ar), hellium (He), nitrogen (N.sub.2), or carbon dioxide (CO.sub.2) gas from the vacuum vessel into the vacuum vessel.
Further, a method is disclosed in U.S. Pat. No. 4,450,186, in which a gas is injected out of the tip part of a mask for preventing a material to be vapor-deposited from depositing on the tip part of the mask to prepare a magnetic tape having uniform magnetic characteristics in the longitudinal direction. Although this prior art discloses that a change in magnetic characteristics caused by a change in the incident angle upon the continuous substrate due to deposition of vapor deposition material on the tip of the mask is prevented effectively by the injection of gas, it fails to describe improvement in the magnetic characteristics by the introduction of gas itself.
Further, a method for manufacturing a magnetic recording medium by a vapor deposition method while introducing a nitrogen oxide gas into a cobalt vapor stream is disclosed in Japanese Patent Application (OPI) No. 94135/1983. There has been a strong demand to improve the magnetic characteristics of the magnetic recording medium and improve the friction coefficient when a lubricant is applied to the magnetic recording medium.